You definitely should avoid the 6.4L for diesel. As with the 6.0, it can be "bullet-proofed", but it's even more expensive to do than the 6.0L. Lots of radiator issues (I had mine replaced), and other issues. I also had turbo seals replaced on mine, and a few other issues. I ended up trading it for a 2011 with the 6.7L The very first 6.7L had some issues that were supposedly fixed in the later builds (after May of 2011 or so I think). I have one of the later builds (mine was built June 2011). I had a DEF sensor issue, fixed under warranty and a turbo cooler oil leak, also fixed under warranty. Otherwise, regular maintenance (batteries, tires, etc.), but no other issues. It has been a great truck and has pulled my 5th wheel all over Texas. Well, east of I-35 anyway.
I don't have any direct experience with the 6.8L or 6.2L gas engines, but they have both been very reliable overall. I had an older 5.4L (2003) that was probably the best truck, no, best vehicle, I have ever owned. 125,000 miles and was never in to the dealer for anything. I would probably still have it, but I needed something better for my 5th wheel.
I would really recommend the 6.7L if you're pulling anything with weight. It is super powerful, quiet, and really a good engine. It does have issues like most newer diesels with all the EPA required crap. But of the engines you are looking at, it is probably the best of them.
Hope this helps a little.